/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineMessageInterface.h
Delegate used when downloading of message contents has completed
typedef TMulticastDelegate_FourParams< void, int32, bool, const [FUniqueMessageId](API\Plugins\OnlineSubsystem\Interfaces\FUniqueMessageId) &, const [FString](API\Runtime\Core\Containers\FString) & > FOnReadMessageComplete